Conventionally, a glass substrate for an information recording medium must have a surface that is as smooth as possible to enable recording of information with high density. Thus, the surface of the glass substrate is ground and polished in plural stages during manufacturing to suppress the formation of microscopic protuberances (refer to, for example, Japanese Laid-Open Patent Publication No. 11-154325). That is, the glass substrate is ground and polished in steps broadly divided into four steps, rough grinding, fine-grinding, first polishing, and second polishing. The type of polishing pads, and the grain diameter and type of polishing agent are appropriately selected in accordance with the required polishing accuracy.
A polishing agent supply device for selectively supplying polishing agents of different types and different grain diameters to the same polishing device has been proposed to change the type or grain diameter of the polishing agent at each polishing step (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-218535). According to such a polishing agent supply device, a plurality of polishing devices do not need to be prepared for each polishing agent that is used. This solves problems, such as that related to installation space.
Recent glass substrates are required to be recordable with higher density. To fulfill this requirement, the smoothness of the glass substrate surface must be improved. In order to improve smoothness of the surface of the glass substrate, the polishing agent must have a finer grain diameter in each polishing step, and the grain diameter of the polishing agent must become finer in a stepped manner as polishing proceeds.
However, when polishing is performed while changing the polishing agent in one polishing device under the above conditions, a problem in which the surface of the glass substrate becomes scratched arises. This is because a polishing agent having a large grain diameter is impregnated in the polishing pad during a preceding polishing step, and the impregnated polishing agent falls out during a subsequent polishing step thereby mixing with the polishing agent having a fine grain diameter. Thus, usage of different types of polishing agents or polishing agents having different grain diameters is impossible in the conventional art, and a plurality of polishing devices must be prepared for each polishing agent when the grain diameter or type is different. Further, when carrying out polishing with a plurality of polishing devices, the task of transferring the glass substrate between each polishing device is troublesome and requires a long period of time.
The present invention focuses on the problems existing in the conventional art. It is an object of the present invention to provide a method for manufacturing a glass substrate for an information recording medium capable of using polishing agents of different grain diameters or different types in the same polishing device, while improving production efficiency. A further object is to provide a glass substrate for an information recording medium having high quality to enable recording at a high density.